Axial Recoil Research Articles

Dissociative electron attachment to carbon tetrachloride probed by velocity map imaging.

Bond-breaking in CCl4via dissociative electron attachment (DEA) has been studied using a velocity map imaging (VMI) spectrometer. A number of effects related to the dissociation dynamics have been revealed. The near-zero eV s-wave electron attachment, which leads to the production of Cl- anions, is accompanied by a very efficient intramolecular vibrational redistribution. This is manifested by a small fraction of the excess energy being released in the form of the fragments' translation energy. A similar effect is observed for higher-lying electronic resonances with one exception: the resonance centered around 6.2 eV leads to the production of fast Cl2- fragments and their angular distribution is forward peaking. This behavior could not be explained with a single-electronic-state model in the axial recoil approximation and is most probably caused by bending dynamics initiated by a Jahn-Teller distortion of the transient anion. The CCl2- fragment has a reverse backward-peaking angular distribution, suggesting the presence of a long-distance electron hopping mechanism between the fragments.

Dissociative electron attachment dynamics of carbon disulfide and violation of axial recoil approximation near 6 eV resonance.

Complete dissociation dynamics of low-energy electron attachment to carbon disulfide have been studied using the velocity slice imaging (VSI) technique. The ion yields of the different fragment anions produced due to the dissociative electron attachment to carbon disulfide for the 5 to 11 eV incident electron energy range have been collected. Two resonances for S- ions are observed at around 6.2 eV and 7.7 eV, while only one resonance for both the CS- and S2- ions at 6.2 eV is present in this energy range. The kinetic energy and the angular distributions of these fragment negative ions at different incident electron energies around the 6.2 eV resonance have been extracted from the velocity slice images. These experimentally obtained angular distributions of different fragment anions combined with previous theoretical calculations provide a detailed picture of the breakdown of axial recoil approximation and the complete dissociation dynamics involved in this resonance.

Effects of π*−σ* coupling on dissociative-electron-attachment angular distributions in vinyl, allyl, and benzyl chloride and in chlorobenzene

We report on a velocity map imaging study of ${\mathrm{Cl}}^{\ensuremath{-}}$ anions resulting from the dissociative electron attachment to four unsaturated chlorohydrocarbons. In all four molecules, this process is mediated by the formation of the lowest shape resonance. The choice of the molecules was motivated by the different character of this resonance. In the planar compounds chlorobenzene and vinyl chloride, it is a ${\ensuremath{\pi}}^{*}$ resonance, which is not dissociative along the C-Cl bond without distortion of the planar geometry. In the nonplanar compounds benzyl chloride and allyl chloride, the shape resonance has a mixed ${\ensuremath{\pi}}^{*}\text{\ensuremath{-}}{\ensuremath{\sigma}}^{*}$ character and is directly dissociative without the need for any additional distortion. Our motivation was to find out whether the dissociation-allowing nuclear motion has a common imprint in the resulting fragment angular distributions. In spite of the expected similarities between the two classes of compounds, the resulting images are quite different for all four molecules. We interpret the results, especially the imprints of the bending dynamics, with the aid of a single-electronic-state model in the axial recoil approximation.

Angle-dependent strong-field ionization and fragmentation of carbon dioxide measured using rotational wave packets

The angle-dependent single ionization of carbon dioxide by linearly and circularly polarized pulses is investigated experimentally. The authors show that delay-dependent ion-momentum measurements from rotational wave packets can be used to test the validity of the axial recoil approximation.

Progress of ultrafast dissociation dynamics for the hydrogen molecule ions

As the simplest molecules with only one electron and two nuclei in nature, H2+ and its isotopes are of special importance and serve as benchmark systems for ultrafast science and technologies. Their dissociation in strong laser fields has attracted a lot of interests in past decades because it directly relates to the production of chemical reactions, and a series of great achievements have already been made successively under the cooperative efforts between the theoretical and experimental physicists. With the rapid technological development of lasers as well as the advanced experimental techniques in recent years, some new trends in this topic are presented. In this paper, we focus on recent research progress of experimental and theoretical works on hydrogen molecular ions dissociation. We first give a background information about past achievements, and then introduce general numerical methods to deal with dissociation, i.e., the numerical simulation of the time-dependent Schrodinger equation (TDSE), and the combination of the strong field approximation and TDSE simulation. The latter was developed to investigate dissociative ionization of hydrogen molecules. Next three related hot research topics are reviewed sequentially, including interference between dissociation channels, photon energy sharing between electron and nuclei, and molecular rotation in dissociation. In detail, some strategies have been raised to control the unique electron localization after the molecular dissociation, which is due to the interference between the lowest two electronic states with opposite parities. In addition, the pump-probe technique was also utilized to observe electron localization dynamics in real time. However, interference of nuclear wave packets carrying different angular momenta in the same electronic state and ending with the same momentum contributes novel spiral momentum distribution under a circularly polarized laser pulse. The electron-nuclear correlation for different ionization mechanisms is mainly indicated in the joint energy spectra, which was extracted and explained by coincident measurement and various theoretical methods. The molecular rotation is generally assumed frozen in dissociation process, which is the common axial recoil approximation. However, recent theoretical and experimental researches show that the ultrafast electron dynamics may play roles in the molecular rotation though their timescales are significantly different. In conclusion, the studies of hydrogen molecular ions not only further deepen our understanding of ultrafast dynamics in small molecules, but also shed light on observing, understanding, and ultimately controlling complex chemical reactions.

Triple ionisation of OCS: orientation dependence in the case of ion impact

The dependence of the cross-section for multiple ionisation on the orientation of free molecules with respect to the incident projectile has long been demonstrated in the case of diatomic molecules. The dependence has been studied via the coincidence momentum imaging technique, where, under the axial recoil approximation, the orientation of the molecule with respect to the incident projectile is determined from the measured momentum vectors of fragments, which, for diatomic molecules, are necessarily emitted back-to-back. The case of triatomic molecules is complicated owing to the possibility of simultaneous or step-wise breaking of the two bonds, precluding a simple mapping of the fragment momentum vectors to the initial orientation. We report a measurement of triple ionisation of an asymmetric triatomic molecule, OCS, which clearly shows, that the process indeed has an orientation dependence similar to that in diatomic molecules. As expected from the studies on diatomic molecules, the dependence is stronger for small values of q/v and negligible for large values of q/v, where q and v are the projectile charge and velocity, respectively.

Attosecond angular streaking on H2 with all-ionic fragments detection

SynopsisWe explore a novel attosecond angular streaking scheme on H2 based on the all-ionic dissociative fragments detection. We aim to reveal the same information from the ionic attoclock as extracted from conventional attoclock measurements [1]. We examine the validity of the axial recoil approximation questioned in [2]. The advantage of the proposed scheme is that it does not rely on the adiabatic electron tunneling hypothesis which has raised the ongoing controversy of a finite tunneling time in the attosecond community [3]

Dynamic risk assessment approach of riser recoil control failure during production test of marine natural gas hydrate

AbstractThe marine natural gas hydrate is considered to have huge resource potential in the South China Sea, and a sixty‐day production test has been successfully conducted in 2017. As the crucial and most vulnerable connection of floating platform and subsea wellhead, the risers are of paramount importance in safe production test of the hydrate. Due to harsh environment and the possible drive off and drift off events induced by failure of dynamic positioning system of the platform, the unplanned emergency disconnect of risers is an integral preventive measure to avoid accidents and secure the well. Once the disconnect occurs, the huge potential energy stored in the riser column will release and consequently cause violent axial recoil, which will cause serious consequences if the recoil control fails. This paper presented a quantitative risk assessment approach on the basis of dynamic Bayesian networks to dynamically predict the risk of riser recoil control failure during production test of hydrate. By using the proposed method, the dynamic failure risk of the riser recoil control was figured out, and the most contributory factors were identified. The practical application of the proposed methodology was demonstrated through a case study. Results indicated that the probability of riser recoil control failure can be significantly decreased within one year through equipment repair, and the failure and repair rates of shutoff valves and main air isolation valves are the most contributory factors. Some corresponding safety control measures were proposed to further mitigate the risk of riser recoil control failure during the production test of hydrate.

Orientation dependence of multiple ionisation of a diatomic molecule under proton impact

Fragmentation of multiply ionised molecular ions is investigated using the coincidence momentum imaging technique in an experiment involving proton–CO collisions over the energy range 25–200 keV. From the measured momentum vectors of the fragment ions, the orientation angle—that is the angle between the axis of the molecule and the direction of the projectile at the instant of collision—is determined under the axial recoil approximation. The multiple ionisation yield is found to strongly depend on the orientation angle, giving rise to an anisotropy as well as a forward–backward asymmetry, reflecting the heteronuclear nature of the CO target. A simple model is used to calculate the probability of multiple ionisation, involving Rutherford-like trajectories with an orientation dependent distance of closest approach and an impact parameter dependent single ionisation probability. The computed probabilities show an orientation dependence which agrees closely with experimental observations.

Dissociation dynamics of the water dication following one-photon double ionization. I. Theory

The measurement of the triple differential cross section in the body frame for double photoionization of a molecule can be made in principle by detecting the ionic fragments and the two photoelectrons in coincidence---but only if the dynamics and geometry of dissociation of the doubly charged molecular ion are known. A classical trajectory study of the nine lowest states of the water dication is presented using high quality ab initio potential-energy surfaces. Sampling from a semiclassical initial distribution of positions and momenta is used to approximate ionization from the Frank-Condon region of the ground vibrational state of neutral ${\mathrm{H}}_{2}\mathrm{O}$. Excellent agreement in comparison with preliminary experimental momentum imaging measurements of double photoionization of water show that eight dication states can be unambiguously identified in the experiment with the aid of theory. The theoretical trajectory results allow body frame measurements of double photoionization to yield all eight states even though the usual assumption of direct dissociation, the ``axial recoil'' approximation, breaks down for three of the dication electronic states seen in the experiment.

Experimental and theoretical investigation of the crossover from the ultracold to the quasiclassical regime of photodissociation

At ultralow energies, atoms and molecules undergo collisions and reactions that are best described in terms of quantum mechanical wave functions. In contrast, at higher energies these processes can be understood quasiclassically. Here, we investigate the crossover from the quantum mechanical to the quasiclassical regime both experimentally and theoretically for photodissociation of ultracold diatomic strontium molecules. This basic reaction is carried out with a full control of quantum states for the molecules and their photofragments. The photofragment angular distributions are imaged, and calculated using a quantum mechanical model as well as the WKB and a semiclassical approximation that are explicitly compared across a range of photofragment energies. The reaction process is shown to converge to its high-energy (axial recoil) limit when the energy exceeds the height of any reaction barriers. This phenomenon is quantitatively investigated for two-channel photodissociation using intuitive parameters for the channel amplitude and phase. While the axial recoil limit is generally found to be well described by a commonly used quasiclassical model, we find that when the photofragments are identical particles, their bosonic or fermionic quantum statistics can cause this model to fail, requiring a quantum mechanical treatment even at high energies.

Anisotropy in multiple ionisation of CO by ion collisions at intermediate interaction strengths

Multiple ionisation of the CO molecule following the impact of 100–700 keV He2+ ions was studied. Fragments arising from the dissociation of were recorded using the coincidence momentum imaging technique. By measuring the angular distribution of the fragment ions in coincidence, and applying the axial recoil approximation, the angle between the axis of the molecule and the direction of the projectile was determined. Strong anisotropy was observed for high degree of ionisation. The dependence of the anisotropy on the interaction strength and the projectile energy was studied and the results were compared with the predicted values in the literature for diatomic molecules. Furthermore, an asymmetry in the angular distribution owing to the heteronuclear nature of the molecule was observed and quantified, showing that the anisotropy is not entirely a geometric effect as has been presented in earlier works.

Fully differential study on dissociative ionization dynamics of deuteron molecules in strong elliptical laser fields

Deuteron momentum distributions from the dissociative ionization of ${\mathrm{D}}_{2}$ in intense elliptically polarized laser fields have been explored in a joint experimental and numerical study. The asymmetrical charge localization in the dissociative ${{\mathrm{D}}_{2}}^{+}$ offers a large torque, and thus an elliptically polarized laser field efficiently rotates the molecular ion during its dissociation, resulting in the emission of deuterons finally deviating from the bond direction at the instant of tunneling ionization of ${\mathrm{D}}_{2}$. The rotating torque of the molecular ions increases with the field ellipticity, leading to an ellipticity-dependent tilt angle for the deuteron momentum distribution. Due to the notable rotation of ${{\mathrm{D}}_{2}}^{+}$ during its dissociation, the photoelectron angular distributions in the laboratory frame and the molecular frame are distinct, which illustrates that the axial recoil approximation is broken for discussing the photoelectron angular distributions of molecules in elliptically polarized laser fields.

Mechanical effects on atoms interacting with highly twisted Laguerre-Gaussian light

Laguerre-Gaussian (LG) light modes with large values of the radial index p and the azimuthal index l are considered. We give special emphasis on the mechanical effects on atoms arising from the Gouy phase and the phase due to the curvature of such light. It is shown that the additional phase terms for highly twisted light can lead to mechanical effects based on the substantial reduction of the axial wavenumber. As a consequence, axial recoil effects are diminished in the processes of stimulated emission and absorption of the light by the atoms. Both the scattering force and the momentum diffusion are affected by the inclusion of these normally ignored additional phase terms. The magnitudes of the effects are explored using order of magnitude analysis with typical parameters due to the atoms and the light.

Influence of the initial angular distribution on strong-field molecular dissociation

We study few-cycle, strong-field dissociation of aligned ${\mathrm{H}}_{2}^{+}$ by solving the time-dependent Schr\odinger equation including rotation. We examine the dependence of the final angular distribution, the kinetic energy release spectrum, and the total dissociation yield on the initial nuclear angular distribution. In particular, we look at the dependence on the relative angle ${\ensuremath{\theta}}_{0}$ between the laser polarization and the symmetry axis of a well-aligned initial distribution, as well as the dependence on the delay between the ``pump'' pulse that prepares the alignment and the few-cycle probe pulse. Surprisingly, we find the dissociation probability for ${\ensuremath{\theta}}_{0}={90}^{\ensuremath{\circ}}$ can be appreciable even though the transitions involved are purely parallel. We therefore address the limits of the commonly held ``ball-and-stick'' picture for molecules in intense fields as well as the validity of the axial recoil approximation.

Integral cross section measurements and product recoil velocity distributions of Xe(2+) + N2 hyperthermal charge-transfer collisions.

Charge exchange from doubly charged rare gas cations to simple diatomics proceeds with a large cross section and results in populations of many vibrational and electronic product states. The charge exchange between Xe(2+) and N2, in particular, is known to create N2 (+) in both the A and B electronic states. In this work, we present integral charge exchange cross section measurements of the Xe(2+) + N2 reaction as well as axial recoil velocity distributions of the Xe(+) and N2 (+) product ions for collision energies between 0.3 and 100 eV in the center-of-mass (COM) frame. Total charge-exchange cross sections decrease from 70 Å(2) to about 40 Å(2) with increasing collision energy through this range. Analysis of the axial velocity distributions indicates that a Xe(2+) - N2 complex exists at low collision energies but is absent by 17.6 eV COM. Analysis of the axial velocity distributions reveals evidence for complexes with lifetimes comparable to the rotational period at low collision energies. The velocity distributions are consistent with quasi-resonant single charge transfer at high collision energies.

Dynamics of dissociative electron attachment to ammonia

A joint experimental and theoretical study on dissociative electron attachment to ammonia molecules has identified Feshbach states responsible for the two principal breakup channels observed decades ago. The insight obtained reveals rich collisional dynamics that lead to a breakdown of the axial recoil approximation and to mirror images of the angular distributions in the breakup channels.

Incidence, predictive factors, and clinical impact of stent recoil in stent fracture lesion after drug-eluting stent implantation

BackgroundStent fracture (SF) after drug-eluting stent (DES) implantation was reported to be associated with target lesion revascularization (TLR). We have noted abnormal late acquired stent axial deformation in lesions after DES implantation, especially in SF lesions, and defined it as stent recoil (SR). We evaluated the incidence, predictive factors, and clinical impact of SR in SF lesions. MethodsBetween 2003 and 2012, 5456 patients (11,712 lesions) underwent DES implantations and follow-up angiography within one year after the index procedure. SR was defined as an axial recoil deformation less than 80% of the stent diameter and SF was defined as the separation of stent segments or stent struts. SF and SR were confirmed by follow-up angiography. The primary endpoint was defined as clinically driven TLR. ResultsSF was observed in 494 lesions (4.2%) and SR in 138 of SF lesions (27.9%). According to multinomial logistic regression analyses, severe calcification and ostial lesion in the right coronary artery were stronger predictive factors of SF with SR lesions. The cumulative incidences of any and clinically driven TLR at 5years were both significantly higher in the SF with SR group than in the SF without SR group (51.7% versus 35.0%, P<0.001; 22.2% versus 12.8%, P=0.019; respectively). ConclusionsSR in SF lesions after DES implantation could be related to the lesion characteristics. SF with SR was highly associated with subsequent TLR compared with SF without SR.

Laser-induced Coulomb explosion of 1,4–diiodobenzene molecules: Studies of isolated molecules and molecules in helium nanodroplets

Coulomb explosion of 1,4--diiodobenzene molecules, isolated or embedded in helium nanodroplets, is induced by irradiation with an intense femtosecond laser pulse. The recoiling ion fragments are probed by time-of-flight measurements and two-dimensional velocity map imaging. Correlation analysis of the emission directions of ${\mathrm{I}}^{+}$ ions recoiling from each end of the molecules reveals significant deviation from axial recoil, i.e., where the ${\mathrm{I}}^{+}$ ions leave strictly along the I-I symmetry axis. For isolated molecules, the relative angular distribution of the ${\mathrm{I}}^{+}$ ions is centered at ${180}^{\ensuremath{\circ}}$, corresponding to perfect axial recoil, but with a full width at half maximum of ${30}^{\ensuremath{\circ}}$. For molecules inside He droplets, the width of the distribution increases to ${45}^{\ensuremath{\circ}}$. These results provide a direct measure of the accuracy of Coulomb explosion as a probe of the spatial orientation of molecules, which is particularly relevant in connection with laser-induced molecular alignment and orientation. In addition, our studies show how it is possible to identify fragmentation pathways of the Coulomb explosion for the isolated 1,4--diiodobenzene molecules. Finally, for the 1,4--diiodobenzene molecules in He droplets, it is shown that the angular correlation between fragments from the Coulomb explosion is preserved after they have interacted with the He environment.

Near-UV photodissociation dynamics of CH₂I₂.

The near-UV photodissociation dynamics of CH2I2 has been investigated using a combination of velocity-map (slice) ion imaging and ab initio calculations characterizing the excited states. Ground state I((2)P3/2) and spin-orbit excited I*((2)P1/2) atoms were probed using 2 + 1 resonance-enhanced multiphoton ionization (REMPI) or with single-photon VUV ionization. Two-color ion images were recorded at pump wavelengths of 355 nm, 266 nm and 248 nm, and one-color ion images at the REMPI wavelengths of ∼304 nm and ∼280 nm. Analysis of the ion images shows that, regardless of iodine spin-orbit state, ∼20% of the available energy is partitioned into translation E(T) at all excitation wavelengths indicating that the CH2I co-fragment is formed highly internally excited. The translational energy distributions comprise a slow, "statistical" component that peaks near zero and faster components that peak away from zero. The slow component makes an increasingly large contribution to the distribution as the excitation wavelength is decreased. The C-I bond dissociation energy of D0 = 2.155 ± 0.008 eV is obtained from the trend in the E(T) release of the faster components with increasing excitation energy. The I and I* ion images are anisotropic, indicating prompt dissociation, and are characterized by β parameters that become increasingly positive with increasing E(T). The decrease in β at lower translational energies can be attributed to deviation from axial recoil. MRCI calculations including spin-orbit coupling have been performed to identify the overlapping features in the absorption spectrum and characterize one-dimensional cuts through the electronically excited potential energy surfaces. The excited states are of significantly mixed singlet and triplet character. At longer wavelengths, excitation directly accesses repulsive states primarily of B1 symmetry, consistent with the observed 〈β〉, while shorter wavelengths accesses bound states, also of B1 symmetry that are crossed by repulsive states.